[native] Clean up lines in retryMultimediaMessage

Summary: More fixes I saw while reading this for video work.

Test Plan:
For the nullish coalescing operator, it's the same reasoning as D4873: pendingUploads is an object, so !pendingUploads will only be true when pendingUploads is null or undefined. Thus the nullish coalescing operator covers these cases.

The removal of the else is because of no else after return.

Reviewers: atul

Reviewed By: atul

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D4877